1. Field of the Invention
The present invention relates to an anode that contains an anode active material containing at least one of a simple substance of silicon, a compound of silicon, a simple substance of tin and a compound of tin, a method of manufacturing the same, and a secondary battery.
2. Description of the Related Art
In recent years, many portable electronic devices such as a combination camera (video tape recorder), a digital still camera, a mobile phone, a personal digital assistance, and a notebook personal computer have been introduced, and down sizing and weight saving thereof have been made. Accordingly, as a power source thereof, a light-weight secondary battery capable of providing a high energy density has been developed. Specially, the lithium ion secondary battery in which a carbon material is used for the anode, a complex material of lithium (Li) and a transition metal is used for the cathode, and ester carbonate is used for an electrolytic solution provides a higher energy density compared to traditional lead batteries and nickel cadmium batteries, and therefore the lithium ion secondary battery is in practical use widely.
Further, in recent years, as performance of portable electronic devices has been improved, further improvement of the capacity has been demanded. It has been considered that as an anode active material, tin, silicon or the like is used instead of the carbon material (for example, refer to U.S. Pat. No. 4,950,566). The theoretical capacity of tin is 994 mAh/g and the theoretical capacity of silicon is 4199 mAh/g, which are significantly large compared to the theoretical capacity of graphite, 372 mAh/g, and therefore capacity improvement is expected therewith.
However, a tin alloy or a silicon alloy inserting lithium has a high activity. Therefore, there have been disadvantages that an electrolytic solution is easily decomposed, and lithium is inactivated. Accordingly, when charge and discharge are repeated, charge and discharge efficiency is lowered, and sufficient cycle characteristics are not able to be obtained.
Meanwhile, it has been considered to form an inert layer on the surface of an anode active material. For example, it has been proposed that a lithium salt coat is formed on the surface of the anode active material (for example, refer to Japanese Unexamined Patent Application Publication Nos. 2005-166469, 10-255800, 2005-26230, 2005-142156, 2004-165097, and 2004-171875). Further, the type of anode active material has been considered. For example, it has been proposed that a lithium-containing oxide is contained in the anode active material, or a silicate is used as an anode active material (for example, refer to Japanese Unexamined Patent Application Publication Nos. 2005-011801 and 06-325765). Further, it has been considered to form a layer having ion conductivity and electron conductivity on the surface of the cathode active material. For example, it has been proposed that an inorganic oxide coat is formed on the surface of a cathode active material (for example, refer to Japanese Unexamined Patent Application Publication No. 2007-005267). Further, it has been considered to add an additive to an electrolytic solution. For example, it has been proposed that lithium orthosilicate as a carbon dioxide absorbent is contained in an electrolytic solution, or lithium silicate as an alkali compound is contained in an electrolytic solution (for example, refer to Japanese Unexamined Patent Application Publication Nos. 2006-260889 and 2002-352860). Furthermore, it has been considered to form an inorganic protective film on the surface of the separator. For example, it has been proposed that a lithium salt coat is formed on the surface of a separator (for example, refer to Japanese Unexamined Patent Application Publication No. 2004-158453). In addition, it has been considered to introduce a gas absorbent into a battery. For example, it has been proposed that lithium silicate is included in a battery (for example, refer to Japanese Unexamined Patent Application Publication No. 2004-152619).